Typically when a calling party places a call in an IMS network, an E.164 number associated with a targeted terminal device is translated into a Session Initiation Protocol Uniform Resource Identifier (SIP URI) by a Telephone Number Mapping (ENUM) system. The call is then initiated by a Serving Call Session Control Function (S-CSCF) using the SIP URI.
In instances where the targeted terminal device is a Unified Messaging System (UMS), there may be situations where the terminal is non-responsive due to a failure in operations of said system. Service providers can consider adding a backup UMS which mirror images a primary UMS to mitigate failures. Such implementations, however, can be expensive and complex.